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My 2009 Chevy Aveo 5 with automatic transmission has an issue that started today; on the way home on the freeway, the HOLD light started flashing. I noticed later that the coolant temperature gauge was below "C" as if the engine was ice cold. HOLD light and engine temperature gauge

What could be causing this?

Maybe related: A few weeks ago I was low on coolant; I topped it off and have driven without issue until today. I do have occasional electrical issues, such as the dashboard clock resetting to 01:00 occasionally when the car is off.

Update:

I was able to run real-time OBD thanks to GdD's answer, and coolant got up to 138° F after a ten minute drive, and temperature gauge on dashboard still at "C" so likely wiring/sensor. Also got a pending code of "P0128 Coolant Thermostat Malfunction"

OBD screenshot: OBD showing 138° F

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    I'm wondering if the coolant temp sensor went out. About the only way (besides the gauge blowing up ... not literally) you'd be reading below "C" all the time. Not sure if your car has one or two temp sensors ... some have one for the gauge and one for the computer. Others just feed the computer and the computer controls the gauges. – Pᴀᴜʟsᴛᴇʀ2 Dec 16 '19 at 23:47
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I don't think the clock resetting is related to this, although it could be a symptom of a computer problem. The possibilities that come to mind are:

  1. Sensor failure: the temperature sensor may have failed and be giving a low reading
  2. Sensor wiring problem: this is fairly likely, it all comes down to how electronic temperature sensors (i.e. thermocouples) work: they offer less resistance to electrical current as temperature goes up, i.e. as it gets hotter the voltage on the circuit goes up. Themrocouples are calibrated to give specific voltages at specific temperatures, the temperature readout is simply an indication of the voltage on the circuit. The gauge is at its lowest possible reading, so if you know it isn't right that indicates there is no voltage on the circuit, indicating a break in the wiring or loose connection
  3. Computer failure: the car's computer(s) may have a problem and be misreading the sensor, or displaying the sensor incorrectly
  4. There could be a problem in the instrument cluster electronics

I suggest using an OBD reader to check the engine's temperature as reported by the computer. I use a bluetooth unit with torque on an android tablet personally, but there are many options available. If the car's computer reports a low temperature then it's likely the sensor, if it is giving a P0117 code (low voltage on temperature sensor) then it could be a failed sensor or the wiring.

If OBD shows the temperature is realistic then you have another problem, for instance 3 or 4 above.

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  • Thanks for the suggestions; no CIL yet, but I will plug in my OBD reader to see; I just got a BlueTooth OBD reader so good idea to get what it thinks the running temp is. – Mark Stewart Dec 17 '19 at 14:56
  • Ran the OBD and got a pending code of P0128 Coolant Thermostat Malfunction, and coolant temperature got up to 144° F per OBD after a ten minute drive, and gauge still pegged at "C" – Mark Stewart Dec 17 '19 at 16:03
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To explain for clarity, the HOLD function is to allow the transmission to select a specific gear range by pressing the button by the shifter.

If it's flashing, it probably means there's a condition that's going to keep you from using HOLD mode. Based on resources like this, it seems temperature malfunctions are one of those conditions so fixing it should solve the issue. If it comes back without any engine codes, you may also have an issue with the transmission.

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